The use of portable electronic equipment has increased as the devices have become lighter and smaller, due to recent developments in the electronics industry. Accordingly, research on batteries as a power source for this portable electronic equipment with a higher energy density has also increased.
A rechargeable lithium battery is fabricated by filling an organic electrolytic solution or polymer electrolyte between positive and negative electrodes. The electrodes reversibly intercalate and deintercalate lithium ions, and produce electrical energy through oxidation and reduction reactions during the intercalation and deintercalation of the lithium ions.
Lithium has commonly been used as the negative active material for rechargeable lithium batteries. Carbon-based materials such as crystalline and amorphous carbon have been replacing lithium metal because lithium causes short circuits in the batteries as a result of the formation of dendrites, and can also bring about explosions.
The positive active material is known to play the most important role in safety and performance of a rechargeable lithium battery. Because of this, chalcogenide compounds or complex oxide materials such as LiCoO2, LiMn2O4, LiNiO2, LiNi1−xCoxO2 (0≦x≦1), LiMnO2, and so on are being studied. Cobalt-based materials, including LiCoO2 are the most widely examined as a positive active material due to high energy density (theoretical capacity of 274 mAh/g with LiCoO2) and excellent cycle life characteristics (capacity retention).
However, due to its structural instability, only about 50% of the theoretical capacity of LiCoO2, which is approximately 140 mAh/g at a charge voltage of 4.2V, is practically obtained that is to say, the non-utilized amount of Li in LixCoO2 is over 50% (x>0.5). In order to obtain more than 50% of the theoretical capacity of LiCoO2, the charge voltage must be increased to over 4.2V. But the potentially usable x value, which represents the amount of Li in LixCoO2, decreases to under 0.5 when the voltage is increased, and a consequential structural instability due to a phase transition from a hexagonal to a monoclinical structure also sharply decreases its capacity retention.
As a result, the search continues for a positive active material for a rechargeable lithium battery that has a high energy density at a high charge voltage of greater than 4.2V, and excellent cycle life. For example, LiNixCo1−xO2 (0≦x≦1), LiNixMn1−xO2 (0≦x≦1), Li(NixCo1−2xMnx)O2 (0≦x≦1), LiCoO2, and LiNiO2 derivatives where elements such as Ni, Co, Mn, and so on are modified (Solid State Ionics, 57,311 (1992), J. Power. Sources, 43-44, 595 (1993), Japanese Laid-Open Patent Pyung 8-213015 (Sony Company (1996)), U.S. Pat. No. 5,993,998 (Japan Storage Battery) (1997)) are being examined. However, these positive active materials have a disadvantage of poor inhibition with regard to swelling at high temperatures.